(i) Field of the Invention
The present invention relates to a cold rolled steel sheet of low carbon-aluminum killed steel, and a method of making the same, and to a hot-rolled steel strip from which it is made. More specifically, the present invention relates to a cold rolled steel sheet having good deep drawability and anti-aging properties, and its manufacturing method together with a hot rolled steel strip of which it is made.
(ii) Description of the Related Art
Since a cold rolled steel sheet has higher dimensional accuracy, finer surface appearance and more excellent workability as compared to a hot rolled steel sheet, a cold rolled steel sheet is widely used for automobiles, electric appliances, building materials and the like. Heretofore, mild cold rolled sheets having higher ductility (a total elongation:El) and Lankford value:(r-value) have been proposed as cold rolled steel sheets having good workability. These steels utilize adjustments of various compositions of steel, or a combination of compositions and manufacturing methods. A typical example is an extra low carbon steel sheet in which the amount of C in the steel is reduced to 50 ppm or less in the steel making process, and to which an element forming a carbide and a nitride (such as Ti and Nb) is added. These steel sheets are mainly manufactured by continuous annealing. Such a steel sheet can achieve excellent characteristics such as a yield strength (YS) of .ltoreq.200 Mpa, a total elongation (El) of .gtoreq.50% and an r value.gtoreq.2.0. Additionally, in such an extra low carbon steel sheet, the solute carbon and the solute nitrogen, which tend to cause aging deterioration, are completely stabilized as carbide or nitride. Therefore, material deterioration is scarcely caused due to aging by solute nitrogen or by solute carbon.
However, as described above, the extra low carbon steel is produced by degassing in order to reduce the amount of C to 50 ppm or less. Thus, the production cost of the extra low carbon steel is higher than that of common low carbon killed steel: 0.02%-0.06%). Furthermore, the characteristics of the extra low carbon steel sheet other than workability are inferior to those of common low carbon killed steel, more specifically, chemical conversion treatability, welded joint strength or the like as disclosed in "TETSU-TO-HAGANE" ((1985)-S1269) edited by the Iron and Steel Institute of Japan and "Current Advance in Material and Process" (Vol. 1, (1988)-946) edited by the same. Accordingly, there are many applications for which only low carbon killed steel must be used.
However, when the low carbon killed steel is used as the source, it is not easy to manufacture a cold rolled steel sheet having good workability and anti-aging properties by continuous annealing. In general, the temperature after hot rolling is 600.degree. C. or more, in order to fix the solute nitrogen as AlN. In continuous annealing after cold rolling, rapid cooling is performed in the cooling process, after completion of recrystallization. Then, while holding the sheet for a few minutes at a temperature of 300-500.degree. C., cementites precipitate in the crystal grain and the grain boundaries, and this reduces the amount of solute carbon. Even in such a method, it is very difficult to manufacture a steel sheet having good anti-aging properties, in which the aging index is 40 Mpa or less. (A.I.: after a tension of 7.5%, the tensile stress difference before and after aging treatment for thirty minutes at 100.degree. C.).
Moreover, as described above, an important factor in making a cold rolled steel sheet having excellent workability is the provision of an extra low carbon steel sheet. Accordingly, in recent continuous annealing facilities averaging treatment facilities are considered to be metallurgically unnecessary. Furthermore, due to problems such as construction cost, averaging treatment facilities are not always provided. When the low carbon content killed steel passes through the continuous annealing facilities, it has been found to be impossible to manufacture a steel sheet having an A. I. (aging index) value of not more than 40 MPa.
In order to obtain a product having good anti-aging properties by applying averaging treatment for a short time, study and development have been undertaken. In the method proposed in Japanese Patent Application Laid-open No. 57-126924/1982, after completion of hot rolling of a steel containing C and Mn within a predetermined range, the steel is coiled at 400.degree. C. or less. The resulting cementite is finely dispersed in the hot rolled steel sheet. The very fine cementite serves as a precipitation nucleus (precipitation site) for the solute C so as to reduce the amount of solute C. Moreover, in the method proposed in Japanese Patent Application Laid-open No. 2-141534/1990, an appropriate hot rolling condition including slab heating temperature is determined for the low carbon killed steel to which a little more Al and N are added, or for a steel to which B is added. The solute N in the steel is completely fixed as AlN or BN. The AlN and BN are defined as a precipitation nucleus (precipitation site) so as to precipitate the solute C and to perform temper rolling at a high reduction ratio.
However, in the method described in Japanese Patent Application Laid-open No. 57-126924/1982, since the coiling temperature is low, the crystalline grain is fine. Therefore, increase of strength (YS) and reduction of workability (El) cannot be avoided. Furthermore, in the method described in Japanese Patent Application Laid-open No. 2-141534/1990, although a cold rolled steel sheet with good anti-aging property can be obtained, temper rolling at a high reduction ratio is essential. Accordingly, increase of YS (yield strength) and reduction of El (elongation) are also caused. In any known method, it is difficult to obtain both excellent workability (more specifically, ductility) and excellent anti-aging properties.